A DC—DC converter is a DC power supply that is small, lightweight, and highly efficient, and uses a semiconductor switching element. It is widely used in electronic devices, etc. In recent years, there has been a great demand for small, lightweight, and highly efficient power supplies. The basic principle of a DC—DC converter is that the switching element is turned on/off at high frequency, and the ratio of the on-time to off-time, that is, the duty ratio, is controlled in a variable manner to keep the DC output voltage at a certain level. Usually, the voltage is controlled to a constant level through negative feedback of the output voltage. More specifically, the output voltage is fed back and compared with a reference value by an error amplifier to find the error. A switching control circuit varies the duty ratio of the switching drive pulse sent to the switching element corresponding to the error signal of the error amplifier.
In a conventional DC—DC converter, in order to protect the load circuit from over-voltages, when the DC output voltage exceeds a preset monitoring level, a protection circuit comprised of a comparator or Zener diode, etc. starts to operate to stop the switching control operation in the control system (specifically, the switching control circuit). In order to restore regular operation after the switching control operation is stopped, a start command can be applied from the outside after the output voltage drops to zero V, or the switching control operation in the control system is restarted after it is found that the output voltage goes below than a prescribed level, such as the aforementioned monitoring level.